Conventional phenol-formaldehyde resins fall into two general classes, namely, the novolacs and the resoles. Novolacs are thermoplastic materials and require the addition of cross-linking agents, such as hexamethylene tetramine, for curing, whereas resoles are thermosetting materials.
Novolacs are produced by reacting less than 1 mole of formaldehyde per mole of phenol. In one process, the reaction is carried out under reflux conditions in a strongly acid medium. The product obtained has the structure: ##STR1## This material contains methylene linkages which are randomly located ortho and para to the phenolic hydroxyl groups and no reactive end groups, so that the novolacs are thermoplastic and to use them as thermosetting materials, they are compounded with cross-linking agents.
In an alternative procedure for the production of novolac resins, phenol is reacted with formaldehyde in the presence of water soluble metal carboxylates as catalyst under mildly acid conditions. An intermediate of the following approximate structure is formed: ##STR2## This intermediate contains benzyl ether linkages which are mainly ortho to the phenolic hydroxyl groups and is produced along with unreacted phenol.
The mixture of intermediate and unreacted phenol then is heated to a temperature above about 160.degree. C. to produce a novolac resin having the structure of formula 1, except that the resin has mainly ortho substitution.
Resoles are prepared by reacting more than 1 mole of formaldehyde with 1 mole of phenol in an alkaline reaction medium. The resins produced have the approximate structure: ##STR3## These materials contain methylene linkages which are randomly located ortho and para to the phenolic hydroxyl groups in the same way as the novolacs of formula 1, but contain reactive end groups, so that the resoles are thermosetting.
Phenol-formaldehyde resins of both types are extensively used industrially owing to their versatility, low cost, extreme stability to weathering, good adhesion to a variety of substrates and stability at relatively high temperatures in the fully cured state.
Resole resins of the above formula 3 in the uncured form have only a limited shelf life and other limitations which have lead to attempts to produce thermosetting phenol-formaldehyde resins of improved properties.
In Canadian Pat. No. 927,041, there is described a particular procedure for the production of thermosetting phenol-formaldehyde resins by reacting formaldehyde with phenol at a mole ratio of at least 1:1 in an essentially aqueous reaction medium in the presence of a water-soluble metal carboxylate catalyst while in copending U.S. application Ser. No. 779,622 filed Mar. 21, 1977, now abandoned, by Sim Y. Leong, Pitchaiya Chandramouli and Ramesh C. Vasishth entitled "Production of Thermosetting Phenol-Formaldehyde Resins" and assigned to the assignee of this application, there is described an alternative procedure for the production of the same resins.
The product resins formed by the procedures of Canadian Pat. No. 927,041 and the aforementioned copending U.S. application Ser. No. 779,622 have a long shelf-life and are characterized by a structure in which a large proportion of the benzene ring linkages are benzyl ether linkages located ortho to the phenolic hydroxyl groups, i.e., a structure very similar to the intermediate of formula 2 formed in novolac production, that is, even when less than one mole of formaldehyde per mole of phenol is used.
Two distinct steps are required in the formation of resins of the resole type and of the latter type. The first step is the exothermic addition of formaldehyde to phenol to form methylol phenol ("methylolation") and the second step is the polymerization of the methyl phenol by condensation to form the thermosetting resin.
While the two steps are common to the two processes, the precise chemical reactions are quite different. Thus, while the addition of formaldehyde to phenol in the conventional resole process produces a randomly substituted ortho- and para-methylol phenol, the addition step in the procedures of Canadian Pat. No. 927,041 and the aforementioned U.S. application Ser. No. 779,622 produces mainly ortho-substituted methylol phenol. Similarly, while the polymerization step in the conventional resole process produces a product with methylene linkages between the benzene rings, the polymerization step in the procedures of Canadian Pat. No. 927,041 and the aforementioned U.S. application Ser. No. 779,622 produces a resin in which a large proportion of the methylene linkages are replaced by benzyl ether linkages. Since the chemical reactions occurring are different, the processing variables affecting one process do not necessarily affect the other process in the same manner.
In Canadian Pat. No. 927,041, it is recited that the procedure for the production of the resin must be carried out in a particular manner. Thus, it is said that it is essential to the process of Canadian Pat. No. 927,041 to carry out the process at a temperature between about 60.degree. and about 80.degree. C. during the initial exothermic reaction of formaldehyde with phenol in order to control and dissipate the considerable exotherm and prevent a runaway reaction. Once the exotherm has subsided, the reaction mixture is heated to about 90.degree. to 100.degree. C., typically at about 90.degree. C., to carry out the polymerization step and form the desired product.
In contrast, in the procedure of the aforementioned application Ser. No. 779,622, the resin-forming process is carried out wholly at a temperature above about 90.degree. C. up to the reflux temperature of the reaction medium by controlling the intensity of the initial exothermic reaction of formaldehyde with phenol.